The present invention relates to a reproducing piano, and in particular to an improved method and apparatus for altering the actuator drive during playback to prevent double hammer strikes against the strings.
It is known to record performances of a piano on magnetic tape, for example, and reproduce the performance by replaying the tape and causing the keys to be actuated mechanically. During the record mode, the piano is played by a musician, and sensors detect the timing and velocity with which the keys are depressed or the hammers are moved, and this information is stored digitally in a permanent memory, such as a magnetic tape. During playback, the digital information is retrieved from the tape and converted to analog control signals that energize actuators to play the keys in the same pattern and with the same dynamics as during the original performance.
In U.S. Pat. No. 4,307,648, which patent is incorporated herein by reference, there is disclosed a method and apparatus for measuring the dynamics of a piano performance wherein a shutter is provided for each hammer shank of the piano, as well as a separate optical switch assembly and counter that is responsive to the trigger signals produced as the shutter eclipses the light beam. The counter is responsive to an initiating signal from the optical switch assembly to start the counter and to an end of count signal from the optical switch to terminate the count, the total count defining a time increment. The total count registered comprises a digital signal constituting an inverse function of the near terminal hammer velocity, that is, the velocity of the hammer just before it strikes the string. Digital information corresponding to the count is stored on magnetic tape for recall during playback and reproduction of the original performance.
A microprocessor retrieves the data from the magnetic tape and produces a digital drive value corresponding to the particular key velocity required. A digital-to-analog converter converts the digital drive value to an analog voltage that is proportional to the desired key velocity. A feedback servomechanism circuit comprising a plurality of operational amplifiers and a sense coil is connected to a solenoid and energizes the solenoid with a current that produces a constant velocity. The velocity is maintained constant by means of the auxiliary sense coil within which a permanent magnet connected to the solenoid moves; the coil is connected to the input of the first operational amplifier. This circuit arrangement causes the solenoid to operate as a linear motor with constant velocity, thereby ensuring that transit times and key velocity can be maintained within very close tolerances so that the playback performance is an accurate reproduction of the original performance.
Although the linear key velocity technique on playback is extremely beneficial to accurate reproduction, it generates an unforeseen problem. The action in a piano engages the hammer as the key is depressed until the point of escapement, or "let-off", at which point the action falls away from the hammer and the hammer continues to travel toward the string in free flight. When a human pianist plays a piano, the key is typically accelerated throughout its travel so that at the point of let-off, the key moves with increasing velocity, and this increase is further enhanced because of the reduction in effective mass due to the free flight of the hammer. Thus, the escapement mechanism is very quickly accelerated out of the way of the hammer so that as the hammer rebounds from the string, it does not hit the jack, which is the particular element that throws the hammer toward the string. Thus, the hammer is able to drop back and will not rebound against the string to cause a double strike, as would occur if the jack were still positioned beneath the repetition roller attached to the hammer.
In a reproducing piano of the type discussed previously, the constant velocity solenoid drive causes the key to be depressed with a constant velocity throughout its travel both before and after let-off occurs. When loud note strikes are played back, there is normally no problem because there is sufficient key velocity to move the jack out of the way of the rebounding hammer to prevent double strikes. However, when quieter passages are played and the key is depressed with lower velocity, there is not sufficient velocity after let-off, because of the constant velocity actuation of the key, to cause the jack to be moved out of the way of the rebounding hammer, and the hammer will rebound off the jack and strike the string a second time thereby producing an objectionable echoing effect.
The double strike effect is not a problem in certain prior art reproducing pianos wherein the solenoid is energized with a constant current or constant voltage, because the force produced by the solenoid increases with stroke, thereby causing an acceleration of the key and action after the point of let-off. However, the constant current or constant voltage solenoid that results in a variable velocity drive, cannot accurately reproduce the original performance as can the constant velocity drive system described earlier.
Accordingly, it is desirable to provide a drive system for a reproducing piano having the advantages of a constant velocity drive, yet being able to avoid the double strike effect which is an inherent phenomenon of a constant velocity drive.